nix: openFirewall defaults false across forge/gateway/matrix (#651)

mara on #651: "Dont default openFirewall to true."

Flip the `openFirewall` default from `true` to `false` for all three
modules that expose host-side ports:

- `services.hyperhive.forge.openFirewall` (httpPort 3000 + sshPort 2222)
- `services.hyperhive.gateway.openFirewall` (port 80)
- `services.hyperhive.matrix.openFirewall` (httpPort 8008)

Rationale: secure-by-default. With shared host netns, the host +
every agent container reach these services via `localhost` regardless
of the firewall — the open only matters for access from outside the
host. Operators who want external reach now flip the bool explicitly:

    services.hyperhive.gateway.openFirewall = true;

Each description updated to explain the new default + when to flip
it (operator's browser, external git clients, federation announcement,
etc.). Behind a host-level reverse proxy that handles TLS, leave off.

Verified via `nix eval` on a clean stub config:
- forge openFirewall = false
- gateway openFirewall = false
- matrix openFirewall = false
- networking.firewall.allowedTCPPorts = [] (was: [80 2222 3000 8008])

Note: c0re's direct ports (7000/8000/8100-8999) are gated separately
via #621 on `gateway.enable` — that gate stays; this PR only touches
the per-module `openFirewall` knobs.

Closes #651.

nix/modules/hive-forge.nix

@@ -87,12 +87,17 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `httpPort` + `sshPort` in the host firewall. Off when
-        the forge should only be reachable from inside the host.
-        (The container shares host netns, so this is the only
-        firewall layer that matters.)
+        Open `httpPort` + `sshPort` in the host firewall. Off by
+        default (#651, secure-by-default): the forge is reachable
+        from the host + every agent container via `localhost` either
+        way (shared netns), so the firewall opens only matter for
+        access from outside the host. Flip to `true` when you want
+        the operator's browser / external git clients to hit the
+        forge directly. (The container shares host netns, so this
+        is the only firewall layer that matters.)
       '';
     };
   };